Note 4 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash Equivalents	$ 18,432	$ 14,184
Available-for-sale securities	10,573	21,728
Money Market Funds [Member]
Cash Equivalents	2,887	126
Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	10,573
Total assets at fair value	13,460	21,854
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Total assets at fair value	2,887	126
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	10,573	21,728
Total assets at fair value	10,573	21,728
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Total assets at fair value
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash Equivalents	2,887	126
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash Equivalents	$ 2,887	126
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Cash Equivalents
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Cash Equivalents